The present invention relates to a method for making a radiation hardened oxide, and more particularly, to a method for making a radiation hardened device incorporating such an oxide located adjacent an edge.
It is known that a silicon-on-sapphire (SOS) metal oxide semiconductor field effect transistor (MOSFET) with a gate oxide insulating layer grown in a steam ambient at or below about 900.degree. C. is at least partially radiation hard, i.e. changes in the gate threshold voltage are reduced after irradiation by gamma rays as compared to a MOSFET that is not radiation hardened. However, it has been found that oxides grown at these temperatures on a silicon mesa are thinner near the mesa edges than at the center thereof, thereby lowering the breakdown voltage and increasing the tunneling current of the oxide. Oxides grown at higher temperatures are more nearly of uniform thickness, see R. B. Marcus et al. "The Oxidation of Shaped Silicon Surfaces", J. Electrochemical Soc., Vol. 129, (June 1982), pages 1278 to 1282. This reduces the tunneling current and increases the breakdown voltage as compared to oxides grown at the lower temperatures for a given thickness at the mesa center, but such high temperature grown oxides are not radiation hard.